In physical L3 networks, such as the Internet, routers exchange routing and reachability information using various routing protocols, including Border Gateway Protocol (BGP). A primary functionality of BGP is to allow two routers to exchange information advertising available routes or routes that are no longer available. That is, a first router may use this protocol to inform a second router that packets for a given IP address or IP prefix can be sent to the first router. The second router can then use this information to calculate routes.
Within some managed virtualized networks, routes are calculated by a network controller and pushed down to the forwarding elements that handle routing within the managed network. As the controller directs how these forwarding elements will route packets, there is no need for the exchange of routing information between the forwarding elements. However, these managed virtualized networks may send and receive traffic through external networks. This currently requires an administrator to manually provide routes to the routers in the external network.